Paleobiochemistry of algae

Simple morphological as well as molecular fossils have been found in ancient and recent sediments. In most cases these fossils show the typical morphological characteristics of prokariotic algae. Algae have been implicated in the biogenesis of the hydrocarbons found in sediments, a fact which appears to be supported by the nature and distribution of their hydrocarbons. Fifteen different species of algae, including seven Cyanophycophyta (Blue greens), six Chlorophycophyta (greens), one Chrysophycophyta (golden-brown) and a macroscopic marine alga from the coast of Hawaii were analyzed for their hydrocarbon and fatty acid content by gas chromatography and combined gas chromatography-mass spectrometry. All of the algae had significant amounts of hydrocarbons (100 - 3000 ppm) in both the high and medium molecular weight ranges. Normal hydrocarbons ranging from C[lowered 17] to C[lowered 19] have been identified in several cases and n-C[lowered 17] has been found to be predominant in all forms of algae. Some blue green algae contain substantial portions of methyl substituted alkanes. Significant amounts of high molecular weight straight chain olefins and paraffins with maxima at C[lowered 23], C[lowered 27], C[lowered 29] were also identified. Squalene is the only isoprenoid found in these organisms. A triterpene has been tentatively identified in three blue greens. The contemporary representatives of algae found in the sediments analyzed include: seven blue greens, six greens, and a golden brown. A few of these algae show a bimodal distribution of aliphatic hydrocarbons with maxima at C[lowered 17] and C[lowered 23], C[lowered 27], or C[lowered 29]. Similar bimodal distributions of saturated hydrocarbons have been observed in both Tertiary and Precambrian sediments. These observations add support to the interpretation of the biological origin of hydrocarbons found in these sediments. The fatty acid content was very similar in all of the algae analyzed, varying mostly in the degree of saturation. They ranged from 14:0 to 18:4 with the fatty acids, either saturated or unsaturated, predominating in the blue greens, while the C18 fatty acids predominated in the green algae. A macroscopic marine alga from Hawaii and the seed and seed coats of a plant were also analyzed by the same method as the algae. The hydrocarbon distribution of the marine alga was very similar to several of the other algae, as was its fatty acid content. However, the hydrocarbon content of Plantago ovata, similar to higher plants in the overall distribution of high molecular weight alkanes, was different in that one of the major components was an anteiso-C[lowered 18]. P. ovata's fatty acids were similar to those of the algae. Inferences from the distributions of hydrocarbons and fatty acids, plus radioactive labelling studied of algae, suggested something more than a simple decarboxylation or elongation of a fatty acid to produce the hydrocarbons found in these microorganisms. However, labelled stearate appears to serve as a precursor to the n-C[lowered 17] in Chlorella. Attempts of phylogenetic positioning of the algae based on hydrocarbon distributions alone are of little value. On the other hand, there may be some value in using fatty acids in limited taxonomical groups.